Screw spindle vacuum pump and operating method

ABSTRACT

Screw-spindle vacuum pump with at least three closed-off feed chambers located one behind the other along each rotor and method for operating this compressor. The chamber which is last on the delivery side is brought virtually to the compression limit pressure by means of pre-admission, shortly before it opens towards the delivery side, by supplying a pre-admission stream which is at least five times as great as the intake mass stream. A precondition, in this case, is a minimum ratio of external compression to internal compression of five.

CROSS-REFERENCE TO RELATED APPLICATION

This is the national stage of International Application No.PCT/EP98/03544 filed Jun. 9, 1998.

BACKGROUND OF THE INVENTION

The temperature of the gas conveyed by a compressor rises according tothe compression pressure ratio. In screw-type compressors which dependon the least possible play both between the two rotors and between therotors and the casing, the thermal expansion caused on the parts of thecompressor may lead to problems. It is known (DE-A-195 22 559) U.S. Pat.No. 5,924,855, by means of pre-admission, to lower the Temperature ofthe gas contained in the feed cells of the machine. By this is meantadmitting cooler feed medium into the feed cells from a point of higherpressure. The pre-admission quantity supplied in each case to thechambers is small, as far as the efficiency of the machine is concerned.Thus, for example, when a screw-spindle machine is operated as acompressor (U.S. Pat. No. 4,812,110; U.S. Pat. No. 5,082,427), it issufficient for only some of the conveyed gas to be recirculated forpre-admission. Also, when a screw-spindle machine is operated as avacuum pump, it is necessary to comply with different preconditions fromthose occurring when it is operated as a compressor. Firstly, thepressure ratio is disproportionately higher in the vacuum mode than inthe compressor mode, in particular typically well above 100. Secondly,in accordance with this pressure ratio, the temperature reached in theconveyed gas is substantially higher. Finally, it is necessary to ensurethat the achievable vacuum is not impaired by pre-admission backflow.

SUMMARY OF THE INVENTION

The object on which the invention is based is to provide a screw-spindlevacuum pump and a method for operating it which, by means ofpre-admission, allow effective cooling, with the efficiency andachievable vacuum being only slightly impaired.

The solution according to the invention is found in the features setforth herein. These presuppose a screw-spindle vacuum pump which, alongeach rotor, has at least three feed chambers located one behind theother. The latter are in each case closed off, with the exception of theplay which is unavoidable in the case of dry conveyance. In such amachine, there is provision, according to the invention, for the chamberwhich is last on the delivery side to be brought virtually or completelyto the compression limit pressure by means of pre-admission, shortlybefore it opens towards the delivery side, by admitting a pre-admissionstream of cool gas which is at least five times greater than the intakemass stream. In this case, an operating point is presupposed, at whichthe ratio of external compression to internal compression is at leastfive. On he one hand, effective cooling in the region of the rotorswhich is the most critical for temperature control is thereby achieved.On the other hand, this cooling also has an effect on the penultimatechamber, since some of the cooler gas in the last chamber, the said gasbeing under substantially higher pressure, flows back to the penultimatechamber. Finally, the advantage of this arrangement is that there is aconsiderable reduction in noise being generated, because, when the lastchamber opens towards the delivery side, pressure equalization hasalready been essentially completed. This means that at least 75% of thelimit pressure, preferably 90%, is reached by means of pre-admissionbefore the last chamber opens on the delivery side.

In known machines having a smaller number of chambers, such highpre-admission is not possible, because, due to pronounced leakagelosses, the pressure in the chamber has already risen relatively sharplywhen the outlet is opened, and, consequently, a lower pressuredifference is available for pre-admission.

Also, in this respect, the considerable difference in the pressure ratiobetween compressors and vacuum pumps once again plays a part; owing tothe lower pressure ratio, a relatively higher pressure prevails in thechamber opening towards the outlet in the case of compressors than inthe case of vacuum pumps.

By internal compression is to be meant the ratio of the volumes of thechamber nearest to the suction side, when this chamber closes, and ofthe chamber nearest to the delivery side, when this chamber opens. Ifthe cross-sectional shape of the screw-spindles is constant over theirlength, internal compression is equal to 1.

Another possibility for defining the pre-admission according to theinvention is that the pre-admission volume stream supplied to thechamber which is last on the delivery side, before the latter openstowards the delivery side, is to be greater than 75% of the theoreticalsuction capacity of this chamber at the time of pre-admission, dividedby the internal compression ratio. If pre-admission extends over atimespan of appreciable length, the time at which pre-admission ends isto be taken as a basis. Instead, the mid-point in time between theopening and closing of pre-admission may also be taken as a basis. Thevolume stream must be related to the outlet pressure and to thetemperature of the gas to be admitted. The theoretical suction capacityis the volume of the chamber at the critical time, multiplied by therotational speed.

The hitherto conventional small pre-admission orifices, in which aconsiderable throttle effect is inherent, are not sufficient for thispurpose. According to a rule of thumb, the cross-section of thepre-admission orifice in mm² should be at least as great as thetheoretical suction capacity of the associated chamber in m³/h, butpreferably twice, furthermore preferably three times as great. This, ofcourse, presupposes that the pre-admission orifice, that is to say thewall orifice which introduces the gas into the chamber, is not precededby any narrower cross-sections which once again impair the effect of theorifice width. In this respect, the theoretical suction capacity of thechamber is the product of the volume of this feed chamber, the number ofscrew flights and the rotational speed, the maximum rotational speed tobe expected in continuous operation being taken as a basis.

This definition of the theoretical suction capacity, in contrast to thedefinition given above contains the number of screw flights as a factor.This is explained by the fact that, here, all hose admission orificesare referred to which may be assigned simultaneously to a plurality ofchambers in the case of a multi-flight screw spindle, whereas only asingle chamber is considered above.

The strong pre-admission according to the invention in the last stage isparticularly effective when the screw-flight pitch of the rotors isconstant, that is to say compression theoretically takes placeisochorically. However, in the case of a decreasing pitch, the inventionproves appropriate, since, as a rule, the pitch is never reduced to suchan extent that, even without pre-admission in the last stage, the limitpressure is reached when the pump is at the normal operating point.Moreover, the invention does not rule out also providing weakpre-admission in earlier stages in addition to the strong pre-admissionin the last stage, although this is unnecessary or even undesirable inmost instances of use.

Since the pre-admission according to the invention takes place only inthe last stage and at least three successive feed chambers are provided,the impairment of the suction capacity of the vacuum pump is negligible,provided that the rotational speed is not too low.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to thedrawings wherein:

FIG. 1 is a longitudinal sectional view of a screw-spindle vacuum pumpof the type used in the present invention;

FIG. 2 is an illustration of the 360° circumference of one of the rotorsof FIG. 1 with a pre-admission orifice covered by a face of a screwthread of the rotor;

FIG. 3 is a view similar to FIG. 2 illustrating a different embodiment;and

FIG. 4 is a sectional view taken along the line IV—IV of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION.

Referring now to the drawings in greater detail, FIG. 1 illustrates aknown screw-type compressor as disclosed in U.S. Pat. No. 5,924,855, thedisclosure of which is incorporated herein by reference.

Resting on the foot part 1 is the motor housing 2, which is connected,if need be in one piece, at the top to the flange-like base plate 3 onwhich the pump-chamber housing 4 is mounted the latter is closed off atthe top by a lid 5 which contains a suction opening 6.

Fastened to the base plate 3 are the flange plates 50 of the bearingbodies 7, which in each case serve to carry a rotor 8, the periphery ofwhich has displacement projections 9 which are preferably arranged as atwo-start helix and engage like the meshing of teeth in the deliveryhollow spaces 10 between the displacement projections 9 of the adjacentrotor. In addition, the displacement projections 9 interact at theperiphery with the inner surface of the pump-chamber housing part 4. Therotors 8 are connected at the top to the suction space 11 and at thebottom to the pressure space 12.

The pressure space 12 is connected to a pressure outlet (not shown).These parts are provided at the bottom end of the vertically mountedpump-chamber housing.

Each rotor 8 is connected in a rotationally locked manner to a shaft 20which is mounted at the bottom in the bearing body 7 by a permanentlylubricated rolling bearing 21. A second, likewise permanently lubricatedrolling bearing 22 is located at the top end of a tubular part 23 of thebearing body 7, which projects into a concentric bore 24 of the rotor 8,which bore 24 is open towards the bottom, i.e. on the pressure side.This bearing 22 is preferably located above the center of the rotor 8.The tubular part 23 of the bearing body preferably extends through mostof the length of the rotor 8. In a vertical arrangement of the pump,then end of the tubular part 23 lies substantially higher than thepressure outlet 17. This helps to protect the bearing and drive regionfrom the ingress of liquid or other heavy impurities from the pumpchamber.

Provided in the tubular part 23 of the bearing body are cooling passages25 which are connected via passages 26 to a cooling-water source and viacorresponding passages (not shown in the drawing) to a cooling-waterdischarge. The cooling passages 25 are preferably formed by helicalturned recesses which are tightly covered by a sleeve. The cooling ofthe rotor bearings prolongs the service life of the maintenanceintervals of these bearings if they are permanently lubricated withgrease. Furthermore, the peripheral surface of the tubular part 23 ofthe bearing body is also kept at a low temperature by the cooling. Thisperipheral surface is opposite the inner peripheral surface of thehollow space 24 of the rotor at a slight distance apart. These surfacesare designed in such a way that they are capable of good heat exchangeand therefore heat can be dissipated from the rotor indirectly via thetubular part 23 of the bearing body and its cooling devices 25. Thesurfaces, opposite one another, of the tubular part 23 of the bearingbody and the rotor hollow space 24 may be designed in a suitable mannerin order to improve the heat exchange between them. It is possible tofeed a sealing gas to the rotor hollow space 24 through the bearing bodyor the shaft 20, which sealing gas is Clean copy of amended paragraphsand new claims. discharged with the delivery medium form the pressurespace 12. Apart from the sealing of the bearing region, it can alsoserve to additionally cool the bearing, the bearing body and the rotor,but in this case it is expediently not directed through the bearing orbearings in order not to contaminate the latter but is directed via apassage 28 forming a bypass.

The delivery effect can be brought about by the gap between rotor andbearing body widening conically towards the pressure space. Here, thegap width (distance of the surface of the bearing body from the surfaceof the rotor) remains essentially constant. In addition, the surfacesopposite one another may also be provided in this case with a deliverythread on one side or both sides, but this is not necessary.

Since the equipping of the gap between rotor and bearing body with adelivery thread or conicity acting in a delivering manner provides avery effective seal against the ingress of liquid or solid particles,additional sealing devices may often be dispensed with; however, theymay be provided, and in fact preferably in a non-contact orminimum-contact type of construction, e.g. labyrinth seals orpiston-ring-like seals.

On account of the sealing action of the delivery thread or the gapconicity, the pump according to the invention is insensitive to thepresence of liquid in the pump chamber as long as the rotors arerotating. This insensitivity also exists in the stationary state owingto the high bearing arrangement in the rotor as long as the liquid inthe pump chamber does not reach the bearing level. It is not onlyimportant when the delivery medium carries a liquid surge with it butmay also be utilized for cleaning and/or cooling the pump by liquidinjection. For example, cleaning or cooling liquid can be injectedthrough nozzles, of which one is indicated at 27. The same or separatenozzles 27 may be used for injecting the cleaning liquid and the coolingliquid. The delivery action in the gap between rotor and bearing bodymay also be utilized to deliver sealing gas independently of an externalcompressed-gas source. However, to deliver the sealing gas independentlyof a external compressed-gas source. However, to deliver the sealinggas, the action of such a compressed-gas source will generally bepreferred in order to feed the sealing gas independently of the rotorspeed. Cooling of the housing shell is not necessary in all cases.However, in the context according to the invention it is advantageouslypossible, since the rotors 8 are also cooled and their thermal expansionis therefore limited. It need not be feared that the rotors run againstthe housing only because they expand, while the housing is kept at alower temperature.

The pump according to the invention may be provided with pre-admission.This means that passages 31 are provided in the areas of higher, orpossible even average, compression in the housing, through whichpassages 31 gas of higher pressure than corresponds to the compressionstate in this area of the pump chamber is let into the pump chamber inorder to effect cooling and/or noise reduction according to knownprinciples. According to an advantageous feature of the invention, thepre-admission gas can be extracted directly form the pressure side ofthe pump by being cooled.

The rolling bearings 21, 22 in the example shown are angular-contactball bearings which are set against one another by a spring 29. Eachshaft 20 carries the armature 35 of the drive motor below the bearing21, preferably directly, i.e. without an intermediate coupling, thestator 36 of which drive motor is arranged in the motor housing 2. Themotor housing may be provided with cooling passages 38.

The flange plates 50, which in the example shown are made in piece withthe bearing bodies 7, are mounted with their outer margins 51, whichessentially follow the periphery of the pump-chamber housing 4, andtheir abutting inner margins 52 on the top side of the base plate 3. Theflange plates 50 are sealed relative to the base plate 3. The end faces53, which follow a secant in radial section and at which the flangeplates 50 bear against one another, are also provided with a sealinginsert.

A turned recess is provided below the flange plates 50 between themargins 51, 52 which turned recess encloses with the top side of thebase plate 3 a space 39 which serves to accommodate synchronization gearwheels 40 which are arranged in a rotationally locked manner with knownmeans on the shafts 20 between the bearings 21 and the motor armatures.So that they can mesh with one another in the area of the inner margins52 of the flange plates 50, the inner margins have a cut-out at anappropriate point, through which cut-out the gear wheels reach.Remaining below this cut-out on each side is a web to which thereference line of the reference numeral 52 generally designating theinner margin points in FIG. 1. This web is advantageous not only forstability reasons but also because it permits an encircling seal on theone hand relative to the base plate 3 and on the other hand between theflattened secant faces of the flange plates 50.

The turned-out portions 39 in the flange plates 50 have a diameter whichis greater than the diameter of the synchronization gear wheels 40. Theyare arranged with slight eccentricity in relation to the inner margins52 so that the synchronization gear wheels 40 can be inserted uponassembly of the rotor construction units despite the presence of thesealing web at 52.

Since the space 39 containing the synchronization gear wheels 40 iscompletely separate from the pump chamber, there is no risk of thesynchronization gear wheels becoming contaminated. They are merely usedfor the emergency synchronization of the rotors. Their teeth normally donot come in contact with one another. Lubrication is thereforeunnecessary as a rule. Although it may be used if desired, the dryrunning of the synchronization gear wheels simplifies the construction,since sealing between the space 39 and the drive motors is notnecessary.

The synchronization gear wheels 40 may also serve as pulse generatordiscs or may be supplemented by additional pulse generator discs whichare scanned by sensors 42, of which one is shown in FIG. 1. Thesesensors 42 are connected to a control device which monitors therespective rotary position of the rotors relative to a set point andcorrects it via the drive. This concerns electronic synchronization ofthe rotors, which is known as such and therefore need not be explainedin more detail here. The play between the teeth of the synchronizationgear wheels 40 is slightly smaller than the flank clearance between thedisplacement projections 9 of the rotors 8. However, it is greater thanthe synchronization tolerance of the electronic synchronization device.During proper functioning of the latter, therefore, neither the flanksof the displacement bodies 9 nor the teeth of the synchronization gearwheels 40 come in contact with one another. In the event of the latternonetheless coming in contact with one another, they are provided with awear-resistant and if need be slidable coating.

The performance data of the pump, apart from being determined by thedrive output and rotational speed, are determined by the displacement ordelivery volume formed at the rotors and thus by the length of therotors. The delivery data may therefore be altered by altering thelength of the pump part containing the rotors. A series of pumps havingdifferent performance data is therefore preferably distinguished by thefact that the individual pumps of this series differ through graduationof the length of these parts, to which the pump-chamber housing, therotors and if need by the tubular parts, projecting into the rotors, ofthe bearing bodies belong.

It will be recognized that each rotor forms with the associated bearingand drive devices a construction unit which can be mounted independentlyand, apart from the rotor, consists of the bearings 21, 22, the bearingbody 7, the cooling devices provided therein, the shaft 20, thesynchronization gear wheel 40, the associated sensor 42 and the motorarmature 35. These units are inserted into the pump in a completelypreassembled manner. They can easily be removed from the base plate 3 orinserted after removal of the pump-chamber housing. The exchanging ofthese units can therefore be left to the user, whereas the manufacturertakes care of the maintenance of the sensitive units as such.

The pump is preferably of isochoric type of construction so that largerliquid quantities can also be safely delivered.

FIGS. 2 and 3 show the development of the circumference of one of therotors 8. Between the dash-dotted boundaries 70 they show the entirecircumference of one rotor through 360°. The dotted bands show the headsurface 71 of the displacement projections which in FIG. 1 appear asscrew threads 9. Therebetween are the delivery-hollow spaces 10 formedby the grooves between the screw threads 9. The grooves form deliverychambers with are limited by the bottom 72, the flanks 73 of theadjacent screw threads 9 and the inner surface 93 of the housing 4. Theends of each chamber are formed by the mesh with the screw thread of theother rotor. Assuming that the mutual mesh of the rotors takes place atthe dash-dotted lines 70 and is closed at the lines 70 by such mesh. Therotor is supposed to rotate such that its surface moves in the directionof the arrow 78. The chambers 10, therefore, appear to move downwardstowards the pressure space 12. Looking at FIG. 1 it is apparent thereare a number of chambers formed between subsequent threads 9.

Since the rotors with respect to each other and to the housing movewithout contact, there are gaps between the head surfaces 71 of thescrew threads 9 and the inner surface 93 of the housing as well asbetween the surfaces of the screw threads of one rotor and the surfacesof the grooves of the other rotor which are in mutual engagement. As aresult leakage occurs if the pressure in one chamber is higher than thatin the subsequent chamber. If there were no leakage, the pressure ofeach chamber would remain unchanged on the level of the suction pressurefrom the time when it is open to the suction side until it opens to thepressure side or to the pre-admission port 31. Since there is leakage,however, the pressure in subsequent chambers rises during their movementfrom the suction to the pressure side. In consideration thereof they arecalled stages. The pump shown in FIG. 1 has ten stages.

The wall 4 of the casing contains a pre-admission port 79 the contourwhereof is shown in FIGS. 2 and 3. The suction side delimiting edge 80of the pre-admission port 79 runs parallel to the associateddisplacement screw thread 81. The same is true for the pressure sidedelimiting edge 82. The length 83 of the pre-admission orifice in theillustrated embodiments is about ⅖ of the length of the circumference ofthe rotor which means that it is greater than {fraction (1/10)} of therotor diameter.

As far as the width 84 of the pre-admission orifice in the axialdirection is concerned, FIGS. 2 and 3 show different embodiments. InFIG. 2 the width 84 is smaller than the width of the head of thedisplacement screw thread 81. In the embodiment of FIG. 3 the width ofthe orifice is greater.

FIGS. 2 and 3 show the last chamber 74 in a state completely open to thepressure space 12. The subsequent chamber 75 is in the position in whichit is just opening to the delivery side since the end 76 of the screwthread 77 delimiting the chamber 75 on its pressure side is just leavingthe place (line 70) where it meshes with the other rotor in order toclose the end of chamber 75.

The embodiment of FIG. 2 shows that the pre-admission orifice 79 is justcovered by the screw thread 81. In contrast, FIG. 3 shows that thepressure side delimiting edge 82 of the orifice extends still beyond thecorresponding edge 85 of the screw thread 81. The free axial projectinglength of the pre-admission orifice beyond the covering edge isindicated at 86. The distance of the end of the uncovered part of theorifice from the end 76 of the screw thread 77 is shown at 87. Thedistance 86 should be smaller than the distance 87 multiplied by thenumber of revolutions and divided by the sound velocity.

Naturally, the pre-admission time is not sufficient to raise thepressure in the respective chamber up to the pressure in the pressurespace 12. That means that a pressure wave intrudes the chamber 75 whenit opens towards the pressure space 12. For the reasons set outhereinbefore this pressure wave shall not reach the pre-admissionorifice. In the embodiment of FIG. 2 this is clearly avoided because theorifice is closed by the screw thread 81. In the embodiment of FIG. 3 itis prevented too, if the condition is met that the time which thepressure wave needs for the distance 87 equals or is larger than thetime which the edge 85 needs in its downward movement through thedistance 86. If this condition is met, the orifice 79 is closed when thepressure wave there arrives.

In an advantageous embodiment of the vacuum pump according to theinvention, the pre-admission orifice is designed as a slot, in which atleast the delivery-side delimiting edge is designed to be parallel tothe associated displacement screw flight. This affords the advantagethat the slot is open with the largest possible cross-section until thelast possible moment. The slot length is expediently to be greater than{fraction (1/10)} of the rotor diameter, preferably also greater than ⅕.It is expediently of the order of magnitude of one third of the rotordiameter. The width of the pre-admission orifice in the axial directionis expediently between half and the full head width (measured in thesame direction) of the displacement screw flight. It may even exceed thehead width a little, as long as the pre-admission filling of the chamberwhich is last on the delivery side is not put at risk by the connectionalready being made between the ore-admission orifice and the followingchamber.

The suction-side delimiting edge of the pre-admission orifice may alsorun parallel to the associated displacement screw flight. It may be moreexpedient, however, to design the suction-side delimiting edge so as tobe at least partially inclined relative to the associated displacementscrew flight, in order thereby to avoid the pre-admission oririceopening abruptly, which could entail an undesirable generation of noise,in favour of gradual opening. The aim, in general, is to ensure that thepre-admission orifice is closed before the chamber opens on the deliveryside. In other words, the pre-admission orifice is just covered by theassociated screw flight when the rotor is in the position in which thechamber is just opening on the delivery side. This, for example,prevents a pressure surge penetrating into the chamber from the deliveryside during opening from advancing as far as the pre-admission orificeand from driving back into the latter heated gas which would reduce thecooling effect in the next pre-admission operation. Unpleasant noise canalso be avoided thereby. In many instances, however, it is not necessaryfor the pre-admission orifice to be already closed when the chamberopens on the delivery side, provided that care is taken to ensure thatthe pre-admission orifice is closed within that timespan which thepressure pulse emanating at sound velocity from the opening of thechamber on the delivery side would require until the pre-admissionorifice were reached. In other words, the free axial projecting lengthof the pre-admission orifice beyond the covering edge of the associatedscrew flight should be smaller than the distance of the saidpre-admission orifice from that end of the screw flight whichconstitutes the opening of the chamber on the delivery side, multipliedby the rotational speed and divided by the sound velocity.

It is sufficient, in general, if these conditions, which are providedfor avoiding undesirable interaction between the pre-admission orificeand the opening of the chamber on the delivery side, are present at ahigh operating speed (for example, of 6000 min⁻¹) because thesedisadvantages are less significant at lower speeds.

The above statements presupposed that pre-admission is controlled by theinteraction of the pre-admission orifice with the head face of a screwflight. Although this is a preferred version, it should not be ruled outfor the pre-admission orifice to be preceded by valves which areresponsible, or partly responsible in conjunction with the screw flighthead face, for controlling the pre-admission time.

It may be pointed out that the term “pre-admission orifice” or “slot”does not demand that the orifice be undivided. For reasons of productioneconomy, such an orifice may be composed, for example, of a multiplicityof individual bores which are separated from one another by means ofwebs. This affords the advantage that pre-admission may take place byappropriately extending the pre-admission orifice over a greater part ofthe chamber length. In a preferred version, the pre-admission orificecomposed of a plurality of separate part orifices extends over at leasthalf the chamber length. It may amount to up to 270°.

What is claimed is:
 1. A method for operating a screw-spindle vacuumpump having at least two rotating screw spindles extending from asuction side to a delivery side of a pump chamber, each said spindlecomprising a helical screw flight defining at least three conveyingchambers located one behind the other and substantially closed off fromeach other, each said conveying chamber having a theoretical suctioncapacity equal to the volume of said conveying chamber multiplied by therotational speed of said spindles, said pump having an internal pressureand operating in an environment defining an external pressure and havingan internal compression ratio and at least one pre-admission port foreach said spindle, said method comprising the steps of: rotating saidspindles to capture a volume of intake gas in each said conveyingchamber, said conveying chambers advancing along the pump chamber fromsaid suction side toward said delivery side; and admitting a charge ofcooling gas through each said pre-admission port into each saidconveying chamber before each said conveying chamber opens to thedelivery side of said pump chamber, wherein said charge of cooling gashas a volume equal to at least 75% of the theoretical suction capacityof said conveying chamber divided by said internal compression ratio. 2.A method for operating a screw-spindle vacuum pump having at least tworotating screw spindles extending from a suction side to a delivery sideof a pump chamber, each said spindle comprising a helical screw flightdefining at least three conveying chambers located one behind the otherand substantially closed off from each other and at least onepre-admission port associated with each spindle, each rotation of eachsaid spindle capturing an intake mass stream in at least one of saidconveying chambers, said pump having an internal pressure and operatingin an environment defining an external pressure, said internal pressurebeing at least five times less than said external pressure, said methodcomprising the steps of: rotating said spindles to capture an intakecharge having a mass in each said conveying chamber, said conveyingchambers advancing along the pump chamber from said suction side towardsaid delivery side; and admitting a charge of cooling gas having asecond mass through each said pre-admission port into each saidconveying chamber before each said conveying chamber opens to thedelivery side of said pump chamber, wherein the mass of said coolingcharge is at least 5 times larger than the mass of said intake charge.3. A screw spindle vacuum pump comprising: a pump chamber extending froma suction side to a delivery side; first and second displacement rotorshaving at least one screw flight, said at least one screw flight of saidfirst displacement rotor engaging said at least one screw flight of saidsecond displacement rotor to define a series of at least threeclosed-off conveying chambers associated with each of said first andsecond displacement rotors, each said series of conveying chambersextending from said suction side to said delivery side and including alast chamber which is last on the delivery side; and at least onepre-admission orifice for each said displacement rotor, each saidpre-admission orifice having a cross-sectional area measured in mm² andpositioned to admit a charge of cooling gas to each said last chamber,wherein each said conveying chamber has a theoretical suction capacitymeasured in m³/h equal to a volume of the conveying chamber multipliedby a rotational speed of said first and second displacement rotors andthe numerical value of the cross-sectional area of each saidpre-admission orifice is at least equal to the numerical value of thetheoretical suction capacity of each said conveying chamber.
 4. Thescrew spindle vacuum pump of claim 3 wherein said pre-admission orificeis configured as a slot having a suction side delimiting edge and adelivery side delimiting edge and at least the delivery side delimitingedge is substantially parallel to an adjacent screw flight.
 5. The screwspindle vacuum pump of claim 3, wherein said each said displacementrotor has a diameter and said pre-admission orifice is configured as aslot having a length greater than {fraction (1/10)} of the diameter ofthe displacement rotor.
 6. The screw spindle vacuum pump of claim 4,wherein said pump chamber has an axis and each said screw flightradially terminates in a head having a first axial width and saidpre-admission orifice has a second axial width measured between saidsuction side and delivery side delimiting edges and said second axialwidth is between one half and one times said first axial width.
 7. Thescrew spindle vacuum pump of claim 4, wherein each said screw flightradially terminates in a head having a first axial width and saidpre-admission orifice has a second axial width measured between saidsuction side and delivery side delimiting edges and said second axialwidth is greater than said first axial width.
 8. The screw spindlevacuum pump of claim 4, wherein the suction side delimiting edge issubstantially parallel to said adjacent screw flight.
 9. The screwspindle vacuum pump of claim 4, wherein the suction side delimiting edgeis at least partially non parallel to said adjacent screw flight. 10.The screw spindle vacuum pump of claim 4, wherein said pre-admissionorifice is covered by the adjacent screw flight when said last chamberbegins to open on the delivery side.
 11. The screw spindle vacuum pumpof claim 4, wherein said pre-admission orifice has not yet been coveredby an adjacent screw flight when said last chamber begins to open on thedelivery side, resulting in an uncovered portion of said pre-admissionorifice having an axial dimension.
 12. The screw spindle vacuum pump ofclaim 11, wherein said last chamber opens at an opening locationrelative to said uncovered portion so that a pressure wave entering saidfeed chamber at said opening location cannot reach said uncoveredportion before said uncovered portion is covered by said adjacent screwflight.
 13. The screw spindle vacuum pump of claim 12, wherein saiduncovered portion is located a distance along said conveying chamberfrom said opening location and said axial dimension is less than saiddistance along said conveying chamber multiplied by the rotational speedof said first and second rotors divided by the speed of sound.
 14. Thescrew spindle vacuum pump of claim 3, wherein said pre-admission orificecomprises a plurality of bores.
 15. The screw spindle vacuum pump ofclaim 3, wherein said pre-admission orifice comprises a plurality ofbores extending over at least half a length of said feed chamber.